GEOLOGY OF THE NEW YORK CITY AQUEDUCT 93 



larging by ice so far as widening is concerned is practically proven. 

 If may also be overdeepened, by which is meant that it may have 

 been gouged out deeper than could have been done by a stream of 

 water alone. 



If ice action then be granted, the profile ought to be and prob- 

 ably is essentially an ice valley profile, i. e. of a more or less U- 

 shape, rather than of typical stream erosion form. It is certain 

 also in this case, if glacial overdeepening is admitted, that there can 

 be no stream notch in the bottom of it. The significance of this 

 lies in the probability that the floor is approximately the same level 

 on a considerable portion of the bottom, so that when once the 

 margin of this floor is touched the gorge as a whole is thereby- 

 determined for depth. 



After plotting the borings data and relying upon the factors that 

 seem to be most firmly established, it appears that the following 

 statements are as definite as the facts will warrant: 



a The average slope of the Storm King side of the valley above 

 river level is nearly 38°, and this is in several steps or sections of 

 steeper and flatter slopes. The Breakneck side is about the same. 



h The average slope of the Breakneck side of the gorge below 

 present water level (the side on which alone there are enough data 

 to plot a fairly good curve) does not vary much from this same 

 value [see accompanying profile] . And it is also in steeper and 

 gentler slopes, apparently a series of U-shaped forms set one inside 

 the other, each inner one deeper than the next outer one. Each suc- 

 cessive inner step is approximately 300 feet deeper than the last 

 and 1000 feet narrower. 



It is certain that this sort of profile is not as simple as at first 

 appears. The surprising feature is the close approximation of the 

 slopes above and below present river level. In view of the fact 

 that glacial widening has been practically proven, as shown before, 

 not much importance can be attached to this uniformity or simi- 

 larity of slope. Ordinarily such a persistence of slope would be 

 taken to indicate simple stream origin, but having abandoned that 

 hypothesis, the value of the angle as a factor in estimating prob- 

 able total depth is lost. In short, one can not assume that the 

 deepest point is indicated by the intersection of the slopes of the 

 two sides. 



But there is one feature that is at least suggestive. That is the 

 uniformity of the succession of steps and slopes. It was noted 

 above that each successive inner one is about 300 feet deeper and 

 1000 feet narrower. If this uniformity and proportion is main- 



